Fix typo in the GDC-specific implementation of 'copysign' function

[ssvb]

Without this GDC 12.2.0 was failing to compile mir-algorithm:

../.dub/packages/mir-core-1.3.12/mir-core/source/mir/math/common.d-mixin-284:284:175: error: undefined identifier ‘x’
../.dub/packages/mir-algorithm-3.16.12/mir-algorithm/source/mir/format_impl.d:19:24: error: template instance ‘mir.math.common.copysign!double’ error instantiating
   19 |     bool neg = copysign(1, c) < 0;
      |                        ^

[ssvb]

BTW, what's the best way to run a full set of unittests? GDC doesn't seem to be covered by CI at the moment.

[codecov-commenter]

Codecov Report

Base: 39.01% // Head: 39.01% // No change to project coverage 👍

Coverage data is based on head (9a9fba1) compared to base (35d3352).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #72   +/-   ##
=======================================
  Coverage   39.01%   39.01%           
=======================================
  Files          11       11           
  Lines         469      469           
=======================================
  Hits          183      183           
  Misses        286      286           

Impacted Files	Coverage Δ
source/mir/math/common.d	75.00% <ø> (ø)

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[9il]

BTW, what's the best way to run a full set of unittests? GDC doesn't seem to be covered by CI at the moment.

I am not sure GDC support is complete. Likely mir.algebraic could never be supported by GDC because it frequently relies on the new compiler fixups.